1. Field of the Invention
The invention relates in general to electrical equipment and in particular to gas insulated transmission lines having a tensioned insulating means for suspending one or more inner conductors within an outer conductor.
2. Description of the Prior Art
A common insulator used for supporting one or more inner high voltage conductors centrally within the outer conductor of a compressed gas insulated transmission line as well as for supporting any high voltage conductor is the cast insulator which is cast in place around the inner or high voltage conductor or onto a thin metal sleeve which is secured to the inner or other high voltage conductor. A material, such as filled epoxy, is usually selected which has a coefficient of expansion similar to the metal selected for the inner or high voltage conductor so as to minimize the possibility of voids being formed at the critical interface where the insulator meets the conductor. This is because such voids are subjected to high electrical stress at the critical insulator/conductor interface region, which can lead to ionization within the voids, flashover and reduced life expectancy for the insulator. This high voltage electrical field at this critical region approaches a value equal to the product of the field at the inner or high voltage conductor and the dielectric constant of the insulator. From this it can be seen that selection of an insulator material with a low dielectric constant would have advantages. In the prior art, however, epoxy, which has a relatively high dielectric constant of typically four to five, is generally used because it (1) has a coefficient of thermal expansion compatable with the conductor materials to reduce void formations, (2) has necessary mechanical strength, (3) is an arc resistant material, (4) does not carbonize on arcing and (5) can be vacuum cast without internal cavities. Voids at the critical interface region are usually avoided during the production of the insulator provided proper precautions and procedures such as vacuum casting and curing of the cast insulator around the inner conductor are utilized. Other special formulations and proprietary composition materials with lower dielectric constants dielectric constants in the range of approximately two to three, have not been successfully molded or cast around typical inner or high voltage conductors without creating voids since they have much higher coefficients of thermal expansion. Insulators made from these low dielectric constant materials have been produced by injection molding methods, and have been used successfully as insulators in gas insulated equipment provided the insulator utilizes a conducting layer at the conductor interface and/or has shielding rings proximate this region. Metallizing the insulator interface surface is expensive and the metallization is subject to flaking, the voids then formed being subject to the same increased field as at the critical conductor/insulator interface. Other methods such as the use of metallized recesses or nonmetallized recesses with insulating gases introduced in them have been used or proposed to defeat this problem of the high field at this critical region.
Solid insulators such as described above utilized for supporting the inner or high voltage conductor within or spaced from an outer or low voltage conductor are also subject to tracking and treeing, two phenomena due to the high field they are operating in, which cause a conductive carbon path to form on the outside surface or within the solid insulator, thereby destroying the insulator and causing flashover, a short circuit of the gas insulated transmission equipment in which the insulator is used. Accordingly, various methods have been proposed to eliminate the electric field lines going completely through the solid insulators, but rather crossing a gap of the insulating gas utilized for the equipment. Various methods to accomplish this end have included utilizing large gas filled recesses within the solid insulators, radially spacing the inner and outer conductor contact members of the insulator so as to provide an insulating gas gap, and cone or double cone insulators extending from an inner conductor at the apex of the cone along a predetermined angle to the outer conductor at the base of the cone, which predetermined angle is usually greater the more acute the field intensity at the region in question. The disadvantage of these arrangements to provide a gaseous insulation medium along with the solid insulation medium is that the increased electrical property of the solid insulator that is created because of the larger insulating gas gap is accompanied by a corresponding reduction in the mechanical strength of the insulator. A high mechanical strength is necessary to withstand short circuit forces which the gas insulated transmission line or the gas insulated equipment may be subjected to. Accordingly, it would be desirable to have an insulating means for gas insulated transmission lines and other fluid or vacuum insulated equipment for providing gaseous or other fluid or vacuum insulation mediums across an appreciable portion of the radial (or whatever direction for other electrical devices) distance between the low and high voltage conductors without sacrificing the mechanical strength of the insulating means.